Taking the spiral bevel gear transmission system of a helicopter tail reducer as research object, a computation model for the temperature field simulation was built, a steady-state temperature field distribution was derived under normal lubrication condition. The spiral bevel gear can be equivalent to the spur gear in basis in equivalent gear theory, and its gear backlash can be reflected at the transverse plane of the equivalent spur gear .The influence of thermal deformation of the spiral bevel gear transmission system on the gear backlash was computed and analyzed under normal lubrication condition using the thermo-elasticity theory. The results provided a theoretical basis for the design correctness of backlash considering thermal deformation, thus ensuring the working accuracy of the spiral bevel gear.